An approach for modifying the electronic properties of a semiconductor element (e.g. to reduce the resistivity of a semiconductor region in a substrate, so as to change the work-function of a semiconductor gate electrode formed upon this substrate) is to deposit a metal over at least the particular semiconductor element. The stack that includes the metal and the semiconductor material is then heated to produce a semiconductor-metal compound layer. This layer has a lower resistivity than the resistivity of the starting semiconductor material and, thus, has a different work-function. Unreacted metal is then removed from the substrate, such as selectively from the semiconductor-metal compound. Such a process leaves the semiconductor-metal compound layer intact and removes excess unreacted metal from the deposition and heating operations. The semiconductor layers formed by such processes may be referred to as semiconductor metalide layers.
A semiconductor metalide layer that is obtained by selectively removing unreacted metal without performing any subsequent masking step to pattern the metalide layer is typically referred to as a self-aligned structure. In certain embodiments, an additional heating step is performed to further reduce the resistivity of the semiconductor metalide layer, e.g. by changing the crystal phase of this layer.
Examples of such semiconductor-metal compounds are silicides. Metal silicide thin films are commonly used in microelectronic circuits in a variety of applications, such as interconnects, contacts and for the formation of transistor gates. For example, Titanium disilicide (TiSi2), Cobalt disilicide (CoSi2), and/or Nickel silicide (NiSi) are used in Ultra Large Scale Integration Semiconductor devices having submicron feature sizes. As is known, silicide layers have a lower sheet resistance than the corresponding sheet resistance of the silicon from which they are formed.
Due to its electronic properties, germanium is, for various MOS technologies, considered to be a replacement for silicon as the semiconductor material of choice to form substrates and/or gate electrodes. Germanides, e.g. compounds resulting from the reaction between germanium and a metal, such as Ni, are used to reduce the resistivity of source and drain regions, or to reduce the resistivity of gate electrodes and, thus, modify the work-function of the gate electrodes. However, current approaches do not provide for selective removal of unreacted metal to produce self-aligned germanides.